1999 K 3500.Horrible exhaust fumes out of the exhaust. I had the truck checked out at my local mechanic to see if any of the O2 sensors were bad. He told me no codes pulled up. I put new plugs in it about 5,000 miles ago. This truck is used on the farm but would like to get more than the 6 MPG it is getting. What else should I look at or should I take it to the Chevy house and let them use the big computer.$$$$

The first thing I would check is the coolant temperature switch. This device tells the ECM if the engine is cold or up to running temperature. The ECM uses this signal to decide how much fuel to inject. They typically fail in the “cold” mode and the ECM continues to enhance the fuel injection as if the engine was cold. This leads to rich conditions when the engine is warmed up.

Many Chevys use two temperature devices; a switch and a sender. The sender is used for the temp gauge so make sure you address the switch if your model year has separate devices. A cheap device that is easy to change so if you decide to just try it, it won’t break the bank.

Twin Turbo and Tester, Thanks for the tips. The thermostat is fine and the temperature reads accurate to the thermostat I put in a while back. Do I need to still start with the coolant temperature switch or go right to the ECM? Will the computer at the Chevy house issolate the problem even though no codes are coming up?

The coolant temp sensor is a primary input to the computer. And unless it completely fails it might not turn the Check Engine light on. But if it is sending an erronious signal to the computer, the computer believes the signal is valid and will run the engine under the conditions for this erronious signal. Which can be very rich if the sensor has failed where it’s telling the computer the coolant temp is at -10 degrees.

The coolant temp sensor can be measure for resistance to determine if it’s working properly. Or a scanner can be used with monitoring capabilities to check the signal from the temp sensor.